High specific radioactivity ligands for neurotransmitter receptors can be used to characterize in vivo the functional status of the receptor. Ideally this should be done with non-invasive brain imaging techniques, such as emission computed tomography. While resolution of the apparatus improves the study of receptors in several brain nuclei we have decided to prepare appropriate models for these studies. With an ideal ligand, one can study regional distribution, saturability and pharmacological specificity of the binding sites and characterize the various functional states of the receptor. To study DA receptors ex vivo, we have selected 3H-spiro-peridol. This drug is almost an ideal probe for binding studies in vivo because it is poorly metabolized in brain and binds with high affinity to the DA receptors. The data obtained in animal models could be extended to measure DA receptor function in man by injecting spiroperidol labeled appropriately for ECT scanning. To study the GABA receptor, we used 3H-muscimol. Muscimol binds with high affinity and in a saturable manner to GABA-A receptors. Diazepam, 1 mg/kg, increases the Bmax of this binding but fails to change the affinity of 3H-muscimol to different brain areas. In this study we could demonstrate a good correlation between receptor occupancy and behavioral effect of GABA-mimetic.